1. Field of the Invention
The invention relates to a light emitting device and a method for manufacturing thereof.
2. Description of the Related Art
In recent years, researches and investigations on light emitting devices operated by d.c. current have actively been made. Particularly, Group III nitride compound semiconductors represented by gallium nitride (GaN), indium gallium nitride mixed crystal (InGaN), aluminum gallium nitride mixed crystal (AlGaN), and indium aluminum gallium nitride mixed crystal (InAlGaN) have been attracting attention as practical semiconductor materials to be used for light emitting devices such as blue-LED (blue light emitting diode) and UV-LED (ultraviolet light emitting diode).
Conventionally, such Group III nitride compound semiconductors have been formed in single crystal thin films by growing on substrates by MOCVD method (Metal Organic Chemical Vapor Deposition).
However, in the case a light emitting device is manufactured by the MOCVD method, it is required that a substrate to be used has a crystal lattice constant approximately equal to that of a compound semiconductor to be grown thereon and is excellent in heat resistance. That is, there is a problem that the material quality and the size of the substrate are limited.
For example, in the case a Group III nitride compound semiconductor is to be grown, a crystalline sapphire (α—Al2O3) substrate is mainly employed. The sapphire has a crystal lattice constant approximately same as that of a Group III nitride compound semiconductor, particularly, gallium nitride and is excellent in heat resistance and therefore a preferable material as a substrate for MOCVD. However, in the case of using the sapphire substrate, since it is required to carry out the growth on the c-plane thereof, the processability and formability of the substrate are difficult and thus there is a problem of a high material cost.
Further, since it is difficult to form a thin film with a uniform thickness on the entire face of a substrate, a substrate with a large surface area cannot be used and the size is presently about 20 cm×20 cm at maximum and thus there is a problem of low productivity.
Furthermore, since a light emitting device has a multilayer structure made of a phosphor layer (also referred to as an active layer) and an electron/hole carrier transporting layer, it is required to grow a compound semiconductor on a bonding plane of respective semiconductor layers by epitaxial growth without causing crystal lattice strains. The reason is that crystal lattice defects such as dislocation in the periphery of the bonding plane of the respective semiconductor layers are generated and a light emitting efficiency is lowered if crystal lattice strains are caused. That is, it is required to precisely control the crystal systems of the respective semiconductor layers and the lattice constants and it is extremely difficult to set the crystal growth conditions and control the conditions and thus there is a problem of difficulty in manufacture.
Therefore, to materialize a light emitting device capable of carrying out surface light emission, it is tried to perform a method of enabling surface light emission by sandwiching a particle layer in which particles are bonded by calcining between a hole transporting layer and an electron transporting layer and applying voltage between the hole transporting layer and the electron transporting layer. For example, refer to JP-A No. 2001-210865.
However, it is required to bond particles by calcining in a method for manufacturing a light emitting device as described in Patent Document 1 and in the configuration of the light emitting device obtained by the method, therefore there occurs a problem that the method cannot be employed in the case of using a material with a low vapor pressure or a sublimation material such as GaN. Further, since there are voids among particles, the structure is easy to cause current leakage.